Sealing of suction casing on the piston side during a vacuum die-casting method

ABSTRACT

For the use of the vacuum die-casting process, it is an essential precondition that the casting process takes place in a circuit sealed off on the outside, so that the undesirable ingress of air is avoided, corresponding safe sealing systems are necessary. The sealing between a casting piston and a suction box is designed in such a way that a long service life of the seal is ensured and a possible risk of accidents is avoided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to piston-side sealing in die-casting machineswhich work according to a vacuum process, in particular according to theVACURAL process, and are provided with a suction gap or suction box.(“VACURAL” is a German trademark of Maschinenfabrik Müller-Weingarten AGof Weingarten, Germany. Reference is made to U.S. Pat. No. 4,476,911,describing the VACURAL process.)

To increase the functional safety and the service life of the seal,sealing on a casting piston stepped in diameter is proposed.

2. Related Art

The production of light-metal parts is carried out in considerablequantities by the die-casting process. In this case, components made ofaluminum and its alloys occupy a predominant position, but magnesium isbecoming increasingly more attractive as a design material.

The known die-casting processes traditionally carried out are thecold-chamber process or, particularly for smaller components, thehot-chamber process. These processes have proved successful for a widevariety of parts. However, if high-grade components having minimum oxideand impurity inclusions are required, these components ensuring higherdynamic strength values and/or being subjected to a subsequent heattreatment, a vacuum die-casting process is preferably used.

In one of these processes, the metering of the melt into the fillingchamber is effected via a seuction pipe located in the melting bath.This metering of the melt is brought about by the application of avacuum, which is switched on by opening a vacuum valve after the mold isclosed. In addition to this mold vacuum, an additional casting-chambervacuum is also possible for a better and more controlled vacuum effect,as described by the applicant in DE 30 41 340 C2 as VACURAL die-castingprocess.

To achieve corresponding process safety and ensure an optimum vacuumeffect, the entire process must as far as possible take place in aclosed circuit and thus be appropriately sealed off to the outside.

In this case, a region to be sealed off is the rear side of the pistonor of the piston rod which is coupled to the casting unit. The suctionbox which discharges the thin light-metal residues, the “spangle”,escaping on the rear side of the casting piston has proved successful asa seal carrier.

Thus, for example, in EP 0 462 218, a seal which seals off thecasting-piston rod in the outlet region from the suction box is providedin a flanged bush fastened to the end of the suction box. A considerabledisadvantage of this seal arrangement is that, in the event of thepiston breaking off at the connecting thread between piston and pistonrod, the sealing effect at the piston rod is retained. Normally, thecooling-water holes required for the piston cooling are located in thepiston rod, so that, after the piston breaks off, the cooling waterescapes from the casting-piston rod. Due to the situation which thenresults, the cooling water can then run back essentially only via thesuction pipe into the melting bath of the holding furnace. In additionto the disturbance of the casting process and the impairment of themelt, there is then a high accident risk when casting magnesium oraluminum. Possible risks arise due to an explosive-like reaction or dueto the considerable increase in volume and thus the considerableincrease in pressure of the cooling water, meeting the hot melt in thesuction pipe, in the resulting closed-off space. A further disadvantageof this seal arrangement is the increased seal wear if the piston andpiston rod are not symmetrical to one another, i.e. if they are offset.

Sealing on a casting piston has been disclosed by U.S. Pat. No.3,009,218; a casting piston having a uniform piston diameter over itsentire length is proposed. Due to the casting process, damage, such asscouring or scores, occurs in the casting chamber, and this mechanicalimpairment also leads to damage to the casting piston. In the designproposed, the piston seal may then likewise be damaged and the sealingeffect is not ensured in continuous operation.

SUMMARY OF THE INVENTION

The object of the invention is to develop a sealing system for a castingpiston for the use of the vacuum die-casting process, which sealingsystem ensures a long service life and substantially reduces a risk ofaccidents.

According to the invention, the object is achieved in that an extendedcasting piston has at least one step, i.e. a smaller diameter, at itsrear end, and the seal attached in the suction box is in sealingconnection with the piston when the latter is being retracted. In thiscase, the seal is to be fitted via a frictional or positive-lockingconnection, and the sealing ring may have a dividing point which permitsits fitting or exchange without substantial assembly measures. Thecasting-piston design selected, on the one hand, ensures that mechanicalpiston damage does not occur on the smaller seal diameter, since thisdiameter has no contact with the bore of the casting chamber, and, onthe other hand, the casting-piston diameters are concentric due to thesimple manufacturing process. Furthermore, after a piston rod breaksoff, which as a rule would be effected in the region of an advancedcasting position, the cooling water could escape through the annularspace which is now open to the rear.

Further details and advantages of the invention follow from thedescription below of an exemplary embodiment.

The present invention will be described with reference to theaccompanying figures, where:

FIG. 1 shows an injection system of a VACURAL die-casting machine, and

FIG. 2 shows a detail of the casting piston with suction box andsealing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the injection system 1 of a VACURAL die-casting machine,this injection system 1 being connected to the fixed mounting plate 3via an anchor 2. The holding furnace 5 for the liquid melt is arrangedbetween the fixed mounting plate 3 and the column part 4 of theinjection system 1. The suction pipe plunging below the melting bathsurface into the holding furnace 5 is designated by 6. The top end ofthe suction pipe 6 is connected to the casting chamber 7. Located in thecasting chamber 7 is the casting piston 8, which is connected to thecasting-piston rod 9. The casting-piston rod 9 is connected to theactual casting unit 10.

An enlarged representation of an exemplary embodiment of the inventionis shown in FIG. 2. Located in the casting chamber 7 is the castingpiston 8, which has diameters D1 and D2 of different sizes. The smallerdiameter D2 is sealed off by the seal 11 in the retracted positionshown, the retracted position corresponding to the operating state inwhich the melt is sucked in through the suction pipe 6 by the build upof vacuum, whereas sealing is no longer necessary in the forwardmovement of the casting piston 8. The situation may even occur in whichan afterflow of air or the specific use of an inert gas necessitates thelowering of the residual melt and thus the prevention of “freezing” ofthe melt in the suction pipe. The seal 11 is frictionally connected tothe suction box 13 by means of a clamping ring 12. By appropriateshaping of the sealing ring 11, e.g. a T-profile, a positive-lockingconnection may also be selected. A sealing ring 11 divided on one sideor both sides is preferably used, which permit an exchange of thesealing ring only by removing the clamping or retaining ring 12. Theseparable region of the sealing ring may be designed, for example, inthe form of an inclined joint, as a result of which an optimum sealingeffect is achieved. The casting piston 8 is preferably designed in sucha way that the smaller diameter D2 (=sealing diameter) is designed so asto be conically tapered in its end region and the transitions are formedwith diameters appropriate to safely seal the casting piston and thesuction box. This form ensures movement into and out of the seal 11 in afunctionally safe manner, which in turn helps to preserve the seal.

The invention is not restricted to the exemplary embodiment describedand shown. It also comprises all developments by the person skilled inthe art within the scope of the appended claims.

What is claimed is:
 1. A sealing arrangement for a die-casting machinecomprising: a casting chamber; a melting or holding furnace for holdingmelted metal; a suction pipe for effecting transport of metal from themelting or holding furnace into the casting chamber; a casting pistonconnected to a piston rod at a connecting thread for insertion into thecasting chamber, wherein the casting piston has at least two portionswith different diameters D1 and D2, wherein D1 is greater than D2, thesmaller diameter D2 being the diameter of a rear end of the castingpiston; a suction box disposed about the casting piston adjacent saidcasting chamber and removed from said suction pipe, said suction boxadapted to remove metal residues from the casting piston portion withthe greater diameter D1; and a seal having an opening with a diametersubstantially equal to said smaller diameter D2 and being arrangedbetween the casting piston and the suction box to form a seal betweenthe casting piston and the suction box, wherein the seal is in front ofthe piston rod when the casting piston is in a retracted position, andwherein the seal is not in communication with the casting piston portionwith the greater diameter D1.
 2. The sealing arrangement as claimed inclaim 1, further comprising a clamping or retaining ring for connectingthe seal to the suction box in a frictional or positive-locking manner.3. The sealing arrangement as claimed in claim 2, wherein the seal hasat least one separable portion.
 4. The sealing arrangement as claimed inclaim 1, wherein the rear end of the casting piston is formed with aconical end having the smaller diameter D2.